Fun with stats:
Waterdeep: Dungeon of the Mad Mage is ranked #40 out of 71 products with 10 or more reviews, placing it in the 45% percentile. It is rated 1.1 points
higher than the overall average product rating of 75.9%. With 12 reviews, this is the #52 most reviewed product.

There are 2 external reviews of this product with a combined rating of 63%. Read these reviews.

In the city of Waterdeep rests a tavern called the Yawning Portal, named after the gaping pit in its common room. At the bottom of this crumbling shaft is a labyrinthine dungeon shunned by all but the most daring adventurers. Known as Undermountain, this dungeon is the domain of the mad wizard Halaster Blackcloak. Long has the Mad Mage dwelt in these forlorn depths, seeding his lair with monsters, traps, and mysteries—to what end is a constant source of speculation and concern.

This adventure picks up where Waterdeep: Dragon Heist leaves off, taking characters of 5th level or higher all the way to 20th level should they explore the entirety of Halaster’s home. Twenty-three levels of Undermountain are detailed herein, along with the subterranean refuge of Skullport. Treasures and secrets abound, but tread with care!

4 out of 5 rating for Waterdeep: Dungeon of the Mad Mage

I was initially unimpressed by Dungeon of the Mad Mage. While the dungeon was big, it seemed to lack the inventive traps of the Tomb of Annihilation and the driving story of Acererak’s soul monger plan. Fewer things stood out, in part because of the maps and art. While flipping through the book, there was less to catch my eye and pull be to read a level or section.

Once I moved beyond my initial impression, I grew more favourable to this product. This book isn’t what I want, but I think it’s what a lot of other people want. And that’s an important distinction.
This is very much a big book of almost two-dozen dungeons of make your own. Two dozen old school dungeon modules to throw into your campaign as needed or build a story around. The kind of module Grognaria complained was lost with Dragonlance It’s not the book that has such a cool idea and story that you will drop everything to incorporate into your game, but it is the book you might turn to when you need a troglodyte warren or look towards when work is kicking your ass but you still need to plan a session.

If you don’t want a pressing story and ticking clock, this adventure is much more preferable than many of the past adventures, which were so much more story-heavy. If you want to make your own story or hate having to strip out a story, this volume is perfect for you. Especially if you want to return to a simple style of play where the adventurers wander into the dungeon for the sole purpose of seeking treasure, kicking in doors and attempting to turn denizens of a dungeon against each other. Which really seems to FIT the tone of Undermountain. This product tries to stay true to the spirit of Undermountain, rather than trying to reinvent it into something that it’s not. Rather than trying to add a deep story while assigning a complex plot to Halaster. And I can respect that.

4 out of 5 rating for Waterdeep: Dungeon of the Mad Mage

The adventure in itself is pleasant, because the dungeon levels are original and intriguing, and each one has its own factions, themes and secrets. But in the end, spending your time all in the same dungeon is really fatiguing.
Maybe, this product has got good production values because it's a great presentation and resource for Undermountain. You can always use it as a mini-setting for your own campaigns, and use only the levels you like. It has the added value to present challenges up to LVL20 PCs.
Solid link with Dragon Heist. The two books are really intended to be played together!
Art is sparse but of good quality. Maps are clear and useful.

In the end, I'm happy with the book.
Still, as a Megadungeon it's clearly inferior to the best Megadungeon ever, Eyes of the Stone Thief, that truly is a living and customizable environment. Why the authors made some steps back? Couldn't they learn the lessons of the past?

3 out of 5 rating for Waterdeep: Dungeon of the Mad Mage

This is a solid dungeon, with plenty of evocative themed levels. The art is very sparse - header art for each chapter, and a handful of spot illustrations - making it a very text-dense book. If you're after an extended dungeon crawl, this is perfect for you. Full "unboxing" over in the podcast.

4 out of 5 rating for Waterdeep: Dungeon of the Mad Mage

Dungeon of the Mad Mage is an adventure for 5th - 20th level characters and features 23 thematically unique levels (and Skullport). The adventure is this dense sprawling affair and is structured in such a way that in order to finish the campaign characters will need to explore most (maybe 70-90% if you are using xp) of the dungeon. Compare this to Curse of Strahd or Tomb of Annihilation where a group can complete the adventure yet only experience a fraction of the material in the text. The maps in the book, which are very well done, helpfully feature optional passageways where GM's are encouraged to expand Undermountain with their own creations. Considering how much there is to do already a GM would have to be mad as Halaster himself to add more content. To be clear, its overall a great resource. The dungeon levels are interesting, layouts are detailed without getting too bogged down in minutia, and each floor feels unique and special. However, my guess is that in practice most GM's will raid the book for their favorite bits to add to their own adventures and Dungeon of the Mad Mage fills this role admirably. You definitely get your moneys worth, but I think only the most dedicated groups are going to be able to play this adventure from start to finish.

3 out of 5 rating for Waterdeep: Dungeon of the Mad Mage

To describe 23 Levels in a book is quite challenging, and I think this book missed a bit the expectations triggered by various announcements prior publication.

IMO too many save-hooks, with a mediocre explanation why they are there (c'mon why should Durnan or Jhesiyra bother for all these adventurer groups eager to be slaughtered within Undermountain?)

There is a lot of work for the DM to get Undermaountain feeling like one massive structure with ties crisscrossing the levels and City above. Also the lack of UM specific magic Items which in earlier editions abounded, and of course of Spells. So many Wizards living there, why none of earlier edition Waterdeep or Undermountain spells were taken into the book? That and Skullport getting only a few pages, is my motivation to select 3 of 5 rating.

4 out of 5 rating for Waterdeep: Dungeon of the Mad Mage

The content is good, but it could have been presented better.

You know those little blurbs that describe what a room looks like to the players at the beginning of each room description? This book doesn't have them. That is just the general theme about the layout of the entire book. It's easy to miss details unless you thoroughly read the details of each room. There are 20+ dungeons and they are large enough that either going to have to stop and read each description thoroughly and take notes ahead of time or drag gameplay to a halt every time you move to an adjoining room. The details are not always intuitively located.

Example: There is a secret door from one hallway to another to another. There are no details about the hallway. The players find the secret door, open it, and enter the hallway leading to a room. There is a page worth of description of this room, at the end of which lets you know there were plates stacked on the inside of the secret door and the opening the door, breaks the plates, and alerts the creatures in the nearby areas.

That reminds me of the maps. In true 1E style, the map layouts look like they designed either a truly mad mage or a random generator. Sooo much hallway.
"Hey boss, you know were carving this dungeon out stone and earth right?"
"Yes Davis, now dig me a tunnel 100ft west, then 100 south, half long away that one go 50ft east, then 30 ft south, then another 20ft west, putting doors here, here, and here"
"Pretty far apart, those rooms on the other side of those doors must be huge?"
"Not at all Davis my boy, put a 10ft x 10ft closet, on the opposite side of each door"
"Going to keep something volatile in those closets sir?"
"Absolutely nothing actually"

4 out of 5 rating for Waterdeep: Dungeon of the Mad Mage

I'd give it 5 stars but it doesn't have enough art, the maps aren't premium quality for cyber tabletop, and there's not enough plot or adventure hooks. Ultimately I think this is a good product and my players had a great session#1 to experience after their post Dragon Heist shopping spree.

5 out of 5 rating for Waterdeep: Dungeon of the Mad Mage

I quite like this book, every level has its own theme and environment but they're also all connected storywise, so you'll be traversing through a variety of environments and encounter a variety of creatures hacking and slashing your way through the dungeons oldschool style. Though I wouldn't even call Dungeon of the Mad Mage a traditional hack and slash, you could actually talk your way through the encounters. At the end of every level, there's a quick section detailing what happens if the PCs decide to go with this ally or that ally or resolve a situation this way or that way, basically the PCs' choices and actions have consequences. What they do matters.

Something I've found is that while Dungeon of the Mad Mage is not an extensively wide dungeon like it was in previous editions, it's a deep, deep dungeon that seems to be more fleshed out this time round. What's also pretty cool is that the monsters aren't statically locked in place, they're free to wander and roam around and it lends a more organic, lived in feel to the dungeons.

And a further recommendation, using the dungeons from Tales from the Yawning Portal would be an excellent idea and accompaniment, that book actually fleshes out the Yawning Portal moreso than what we get in Dungeon of the Mad Mage, and the dungeons in that book would make for a good fit in Dungeons of the Mad Mage.